1. Field of the Invention
The present invention relates to a non-magnetic single component developing device.
2. Description of the Related Art
The electro-photographic system easily can form images with good image quality and therefore can be used in a wide range of image forming apparatuses such as copiers, printers and facsimiles. The process for forming images using the electro-photographic system includes a charging process in which a photosensitive surface containing a photoconductive substance is uniformly charged, an exposing process in which the photosensitive surface is exposed so that an electrostatic latent image is formed, a developing process in which a developing agent is attached to the electrostatic latent image on the photosensitive surface so that a toner image made of toner contained in the developing agent is formed, a transfer process in which the toner image carried on the photosensitive surface is transferred to a surface of a recording medium such as a paper sheet, and a fixing process in which the toner image is fixed onto the paper sheet by, for example, heating and pressing.
In the developing process, the developing agent is temporarily attached to a developing roller, and this developing agent is supplied to the electrostatic latent image on the photosensitive surface, so that the electrostatic latent image is developed and thus a toner image is formed.
In an image forming apparatus, a portion that performs the developing process is generally referred to as a “developing device”. Although various forms of developing devices are known, non-magnetic single component developing devices, which use only non-magnetic toner as the developing agent, are becoming the main stream. FIG. 2 is a side view schematically showing the structure of a non-magnetic single component device. A developing device 100 includes a developing roller 101, a development bias application power source s2, a supplying roller 102, a regulating blade 103 and a toner container 105. The developing roller 101 is in contact with a photoreceptor 106 in a nip portion (developing portion) 107 and is provided so as to rotate in the direction of an arrow B, that is, in the counterclockwise direction. The development bias applying power source S2 applies a voltage to the developing roller 101. The supplying roller 102 is in contact with the developing roller 101 and provided so as to rotate in the direction of an arrow D, that is, in the counterclockwise direction, to supply a toner 104 to the developing roller 101. The regulating blade 103 is in contact with the developing roller 101 in a portion on the downstream side in the rotation direction of the developing roller 101 from a contact portion 108 where the developing roller 101 is in contact with the supplying roller 102 and forms a toner thin layer on the developing roller 101. The toner container 105 stores the toner 104.
With the developing device 100, first, the toner 104 in the toner container 105 is charged with electricity by friction in the contact portion 108 of the developing roller 101 and the supplying roller 102, and the toner 104 charged with electricity is attached to the surface of the developing roller 101 to which a voltage is applied by the development bias applying power source S2. The toner 104 on the surface of the developing roller 101 is shaped into a thin layer having a uniform thickness while its thickness is adjusted by the regulating blade 103. The thin layer of the toner 104 on the developing roller 101 is conveyed to the nip portion 107 by the continuous rotation of the developing roller 101, and supplied to the electrostatic latent image on the photoreceptor 106, and thus the electrostatic later image is developed. The toner 104 that remains on the developing roller 101 without having been supplied for development of the electrostatic latent image is conveyed back to the toner container 105 by the continuous rotation of the developing roller 101, scratched and collected from the developing roller 101 by the supplying roller 102 in the contact portion 108, and new toner 104 is supplied by the supplying roller 102 onto the developing roller 101 from which the toner 104 has been scratched and collected. Such a function-operation cycle is repeated.
In the image forming apparatus of the electro-photographic system, using the non-magnetic single component developing device advantageously makes it possible to facilitate the maintenance, and reduce the size, the weight and the cost. However, the non-magnetic single component developing device has drawbacks stemming from the material of the supplying roller. More specifically, in general, the supplying roller is formed of foam such as a closed cell foam having closed cells, and an open cell foam having open cells in order to carry toner. However, the supplying roller formed of a signal cell type foam has a very high surface hardness, the pressure by the contact with the developing roller is increased, which increases its driving torque, and thus toner filming on the developing roller, toner degradation, abrasion more than necessary of the supplying roller, and the like can occur easily. In addition, the supplying roller formed of a signal cell type foam can carry toner in an only small amount, so that the amount of toner supplied to the developing roller is decreased, and therefore, defective printing such as blur occurs in the printed images. On the other hand, the supplying roller formed of an open cell foam, in which foam cells are linked continuously, has a relatively low surface hardness, and carries toner in a large amount. Therefore, the performance of supplying toner to the developing roller is good. However, when used continuously for a long time, a large amount of toner, enter inside the open cells and remain there, so that the supplying roller becomes hard, and thus the surface hardness is significantly increased consequently, similarly to the supplying roller formed of a closed cell foam, increase in the driving torque, abrasion in the hardened portion or the like occurs. Furthermore, toner filming on to the developing roller, toner degradation or the like occurs easily. In general, the supplying roller is formed by coating a circumferential surface of a core metal with a foam layer, and when an open cell foam is used for the foam layer, the foam layer is peeled from the core metal in use, because its mechanical strength is insufficient.
On the other hand, in the supplying roller formed of a foam having a large cell diameter, the walls between adjacent cells on the surface of the supplying roller are split as the driving time passes, and thus the number of cells that can temporarily carry toner is decreased. Therefore, the performance of supplying toner to the developing roller deteriorates, the consistency of solid images deteriorates, and solid images are blurred. Furthermore, in the supplying roller formed of a foam having a small cell-diameter, in general, the surface hardness is high, so that the toner filming to the developing loner, toner degradation, abrasion of the supplying roller and the like can occur easily
In view of such a problem, a supplying roller formed of a single cell type rubber foam having a density of 0.18 to 0.28 g/cm3 has been proposed (e.g., JP5-273848A). However, this supplying roller has slightly better surface hardness and performance of supplying toner to the developing roller than the conventional supplying roller formed of a closed cell foam, but its level is not sufficiently satisfactory. Therefore, an increase of the driving torque by continuous use, toner degradation, deterioration of the consistency solid images, and the like cannot be prevented. Furthermore, the durability, in particular, the long term abrasion resistance of this supplying roller is insufficient, and its surface is easily worn away by the friction with, for example, the developing roller, the regulating blade. More specifically, the walls between adjacent cells are split on the surface of the supplying roller, so that the rubber foam is scraped and thus the outer diameter of the supplying roller is reduced from the original size. Consequently, insufficient scratching of toner with the developing roller, a decrease of the charge amount of toner due to insufficient contact, deterioration of the consistency with the developing roller and the like occur. Therefore, defective printing such as blur cannot be prevented. In particular, in heavily printed images such as solid images, printing blur is significant after toner is consumed corresponding to one circumference of the developing roller and one circumference of the supplying roller, more specifically, images in the second half in the printing direction are significantly blurred.
Furthermore, a supplying roller in which a layer made of an open cell foam having a low compressive elasticity and a low hardness is formed on a core metal, and a layer made of a closed cell foam is formed on that layer has been proposed (e.g., JP5-181352A). However, also in this supplying roller, the drawbacks of the supplying roller having a closed cell foam as the surface layer are not sufficiently solved. In particular, the performance of supplying toner to the developing roller is insufficient, so that defective printing such as blur due to long term continuous use may occur easily. Furthermore, for production of the supplying roller, in general, the foaming process for forming a foam layer twice and the abrading process for shaping the foam layer to a desired for twice are necessary, which significantly increases the production cost. Therefore, this is not industrially suitable.
A supplying roller made of a rubber material having a structure in which crosslinking is effected with a plasticizer is used. In this supplying roller, friction and friction heat caused by the contact with the developing roller cause the plasticizer to be oozed from the rubber material so that the crosslinking structure is dismantled, so that the elasticity of the rubber material tends to disappear. As a result, the surface hardness of the supplying roller is increased significantly, and thus the disadvantages as described above are caused.
Furthermore, in an image forming apparatus such as copiers, there is a demand for further increase in the number of sheets that can be oriented with one filling of toner (guaranteed number of sheets), and for this, it is very important to, for example, improve various characteristics of the supplying roller and increase the life of the supplying roller